This invention relates to a system and method of light detection. More specifically, this invention relates to a technique of measuring the intensity of light in order to monitor the intensity of light from a light source and/or in order to measure motion of an object which is being tested.
Optical methods are used in a number of technical fields in order to extract information about a wide variety of systems. As a consequence, a large variety of photodetectors have been developed. Numerous techniques exist which utilize lasers to measure linear and angular displacements or motion. In most applications, linear displacement is best measured using interference techniques. Angular displacement measurements are generally not amenable to interference techniques without adding mass or altering the torsion constant of the system. An example of the latter case is the use of torsional apparatus to measure shear waves and moduli of fibers, polymers, gels, and colloids. For the measurement of small angular deflections in these types of experiments, position-sensitive detectors (PSDs) very often play a large roll in converting the displacement of a laser beam into an electrical signal proportional to the displacement.
Although various PSD designs have been used, they, and the experiments which use them, have been subject to a number of limitations or disadvantages.
Previous PSD designs are usually relatively expensive to produce due to complexities in manufacturing and/or a requirement for a relatively large number of components. For example, a conventional solid state PSD may use a photodiode array. The array has a relatively large number of photodiodes which are used to generate a relatively large number of outputs. By determining which photodiode or photodiodes within the array are being illuminated, one can determine position information relative to the light, this position information being dependent upon the linear or angular displacement or motion of the object under test. Because one needs a sufficient number of photosensitive cells such as photodiodes within the array in order to sense any possible position for the object under test, the array requires a large number of components such that it is relatively expensive.
A further problem which occurs in experiments using light for measurements is that it is difficult to measure the light which is being applied to the object under test at the same time as the test is being conducted. In other words, laser power monitors generally consume 100% of the laser power and cannot be used to measure the laser power while using the laser to do something useful. Currently, laser power monitors are usually calorimeters. They provide the average power, but do not provide the peak power or laser pulsed profile in a pulsed laser.
Although a current high speed photodetector such as Scientec model 301-020 might be used in a crude manner to monitor laser power while allowing use of the laser for application to an experiment, this could only be accomplished by adding several relatively expensive external components.
Therefore, there has been a need for a relatively low cost laser power monitor which can sense the laser power while the bulk of the laser power is being used upon an experiment. Additionally, there has been a need for such a monitor which can provide the peak power or laser pulse profile in a pulsed laser.
The use of a power monitor for measuring the intensity or power of light from a laser or other collimated light source may be useful in a number of different situations. In particular, where the output of an experiment is dependent not only on the object under test, but is also dependent upon the intensity of the light applied to the object, an unstable source of light may introduce inaccuracies into the experiment. In other words, a change in output might be due to changes in the object under test or might be due to variations in the intensity of light supplied from the light source. Under such circumstances, the ability to accurately measure the light from the light source could be used to compensate for variations in the light source. Since traditional photodiode array PSD systems give positional information independently of the incident intensity, these relatively expensive systems would not necessarily require a power monitor. However, such a power monitor may be useful other numerous other situations.